Critical plane approach

When you use the critical plane approach, the durability solver follows these steps:

1. It identifies and processes only the element faces on the surface of the structure or on the displayed group of elements.

2. It calculates the strain time history at the centroid of each element face with respect to the part coordinate system. For static durability analysis, the stress time history is calculated by superposition of static solution results and the load patterns. For transient durability analysis, the stress time history is taken from the transient solution.

3. It calculates the maximum shear strain and principal axes at every time step.

4. It compares the maximum shear strains at every time point to identify the maximum value of the maximum shear strain in the time history. It uses the strain tensor at this point to determine the critical (maximum shearing) plane and the principal axes on the plane, i and j. The k axis is normal to the critical plane.

5. It performs a 2D tensor transformation to calculate the stress histories σi(t) and σj(t) along the two principal axes and the normal stress σk(t).For strain-based fatigue life criteria, the solver also calculates the strain histories in the three principal directions, εi, εj, and εk.

6. It chooses the primary loading axis as follows:For all fatigue life criteria except maximum shear strain life, the primary loading axis is chosen between i and j as follows:The solver calculates the two stress biaxial ratios:r1 = σi/σj**r2 = σj/σiThe solver compares r1 and r2.If r1 is smaller, the direction j is the primary loading direction, and σ1 = σj, σ2 = σi, ε1 = εj, and ε2 = εi.If r2 is smaller, the direction i is the primary loading direction, and σ1 = σi, σ2 = σj, ε1 = εi, and ε2 = εj.The stress biaxial ratio that is used later in the calculations is r = min(r1,r2).For maximum shear strain life, the primary loading direction is the maximum shearing direction (45° from i and j axes). The solver calculates the shear strain history γm(t) in the maximum shearing direction, the corresponding normal strain history εn(t), and the strain biaxial ratio rε = εn(t)/γm(t).

7. It uses the stress ,σ1, or strain, ε1, in the primary loading direction or in the case of maximum shear strain life, the shear strain γm to calculate the stress or strain amplitudes and mean values using rainflow counting. See Rainflow counting and Fatigue life criteria for more information.

8. It calculates damage and life:For uniaxial loading fatigue, it uses the stress or strain amplitudes and means in the S-N curve of the selected life criterion for damage calculation. See Fatigue life criteria for more information.For biaxial loading fatigue, it uses the biaxial ratio r or rε to update the S-N curve of the life criterion for damage calculation. See Biaxial fatigue evaluation on element free faces for more information.

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Static events

Transient events

Analyzing strain gage rosette data

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Fatigue evaluation on element free faces

Principal axes approach

Maximum damage approach

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Critical plane approach, Simcenter 3D 2021.1 Series

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